EAS refers airspeed to the equivilent at ISO sea level.
In absolute still air at all altitudes TAS = ground speed, but EAS becomes progressively less than ground speed as you climb higher and the air gets less dense. It would also reduce as humidity increases for the same reason.
IAS is what the instrument tells you - the number the needle on the dial points at.
TAS corrects for the fact that the barometric instrument reads lower (for a given airspeed) as air density reduces for a given airspeed.
EAS is the number that gives you the "equivalent airspeed" in terms of aerodynamic forces (lift & drag).